Effects of Molecular Weight and Its Distribution on Fractural Behavior of Polystyrene

Abstract

ABSTRACT:The effects of molecular weight and its distribution on fractural properties of polystyrene were studied at several elevated temperatures and various rates of strain. Logarithmic plots of ultimate strength ar against ultimate rate of strain i'r, as well as of ultimate strain rr against rr at various temperatures, can be superposed by horizontal shifting into respective smooth master curves, giving the same shift factors. The shift factors are of the WLF type.Fractural behavior can be distinguished into four regions: brittle to ductile, ductile, ductile to flow, and flow regions. The effect of molecular weight is notable especially in the third and fourth regions. ar and rr plotted against i'r become lower and lower as molecular weight decreases. When a low molecular weight component is included, ar and rr plotted against i'r shift to the short. time-scale (high-rate strain) side. KEY WORDSFracture / Polystyrene / Molecular Weight Distribution / Molecular Weight / Time-Temperature Superposition / The fractural properties of high polymers, especially of vulcanized rubbers and glassy polymers, have been studied by many authors 1 -3 and some results obtained in the field of linear viscoelasticity have been applied to these properties on the basis of the thought that the fractural properties are governed by the some relaxation processes. 4 -s On the other hand, according to Coleman 9 -11 and Kawabata, et al., 12 the fractural properties of fibers and vulcanized rubbers are essentially dominated by stochastic or statistic processes. Recent developments in studies of linear and nonlinear viscoelastic properties of uncrosslinked amorphous polymers are stimulating studies on fractural properties of these polymers. However, there have been only a few studies on effects of molecular weight and its distribution on the fractural properties of these materials.For several years, the authors have devoted themselves to research on the effects of molecular weight and its distribution on the rheological properties of several kinds of polymers such as polystyrene 13 ' 14 and poly(methyl methacrylate). 15 • 16

Extensional and Fractural Properties of Concentrated Solutions of Monodisperse Polystyrene at Elevated Temperatures

“…The stress-strain curves below 84.8°C are different in shape from those above 94.2°C. As was pointed out in a previous paper,2 the change in the shape of the stress-strain curves below 84.8°C results primarily from a decrease of the maximum extensibility, 19 -24 while the change in the shape above 94.2°C results from a counterbalance of two competing processes: (1) the increase in stress due to the extension of molecular chains between entanglement loci, and (2) the relief of stress due to the slippage of polymer chains at entanglement loci. Figure 2 shows that the effect of extension rate on stress-strain behavior is similar to that of temperature, as seen in Figure 1, owing to the role of the friction coefficient, or local viscous resistance.…”

“…The specimen length L varies with time t according to ( 1 ) Therefore, the extension ratio ). is given by J.=~=L0 +vt .…”

The effect of molecular weight and weight distribution upon polymer melt rheology

“…where N * (tt') is the memory function N N* (tt'l = C L, exp (t' -t)/Atl) (1-2) n=l L, = G,/T, (1)(2)(3) A, = T , (1)(2)(3)(4) In E q 1-2, L, is the probability of forming a junction to produce a chain of length n , and A; ' is the probability of destruction of a junction of a chain of length n . The Rubberlike Liquid assumes that L, and A, are constant.…”

“…This model is quite adequate in its description of linear, small strain or small rate of deformation, viscoelastic behavior. If we specify a single relaxation time model, 71 = 7 m u x (1-5) GI = q c J r m u x (1)(2)(3)(4)(5)(6) then the Rubberlike Liquid predicts the following for the material functions in small amplitude sinusoidal shearing and steady shear: Fig. 1 1 .…”

The spinnability of polymer fluid filaments

“…The equation of motion is Equation (28) with the aid of eqs. (13), (14), (16), (19), and (22) reduces to…”